Process for making refractory shapes

ABSTRACT

This application relates to a process for making ceramic shapes from nonplastic size graded ceramic materials comprising the formulation of a unique batch. The batch is processed in a water-cooled pug mill-extruder to prepare a deaired and densified column. The column is cut into constant volume billets. The billets are then pressed or reformed at low pressures into uniformly sized shapes which are thereafter dried and fired to develop a ceramic bond.

RELATED APPLICATION

This is a continuation-in-part of Application Ser. No. 623,741, filedOct. 20, 1975 now U.S. Pat. No. 4,028,453.

This application relates to a process for making ceramic shapes andparticularly relates to a process for making ceramic shapes whichrequire close tolerances and smooth surfaces such as slide gates used inthe teeming of molten metals.

The current process for making slide gates is a combination of impact(vibration) and hydraulic pressing, firing and machining. That processcontains undesirable features, for example, variations from gate to gateare difficult to control. Variations result in high scrap losses and lowproductivity.

A process known as extrusion-repress or the stiff mud process isdescribed, for example, in Refractories, by F. H. Norton, 3rd edition,McGraw Hill Book Company, Inc., New York (1949), pages 123-128. Thestiff mud process has been used in the manufacture of shapes fromplastic batches, that is, those containing a large proportion of crudeclay. They require moisture contents from 8 to 20% by weight of thebatch (or 18 to 40% by volume) to provide an extrudable mass. Even so,in the stiff mud process, the extrusion die is steam heated to reducefriction. Since the extruded or green density of the shapes isinherently low due to the porosity left in the shapes when the water isremoved, high bulk densities must, in the instances, be gained throughshrinkage during drying and firing. However, shrinkage results indistortion of the shapes. Also, the resulting product has unsatisfactoryhot strength due to the inherent impurities and the mineralogicalstructure of the crude clay.

The process according to this invention has numerous distinct advantagesover pressing and other prior art processes for forming refractoryshapes from batches comprising nonplastic materials. Bulk density ofeach shape is high and uniform from end to end, side to side and top tobottom. Dimensional tolerances of the shapes may be closely maintained.Surface quality, detail and overall appearance are improved. Die wearand press maintenance are reduced due to the low reforming pressures andthe elimination of the need for impactors. The process is especiallyadaptable to automatic handling and the elimination of manual labor.

The process according to this invention consists of repressing orreshaping a fully densified extruded preform prepared from a batchcomprising substantially all nonplastic ceramic materials.

According to this invention, high purity, high density, volume stableextrusions are achieved from size graded ceramic raw material batcheswhich contain no clay or only to 5% clay and preferably no more than 2%clay. The extrusions are formed at low moisture levels, for example, 3to 4% by weight of the batch depending on the specific density of theraw materials. The size graded ceramic raw material batch is preparedfrom nonplastic ceramic materials such as calcined alumina or tabularalumina or other granular materials not including raw clays. Actually avery small amount of clay may be added to the batch without changing itsoverall nonplastic character. For example 2% kaolinitic clay or 0.5%bentonite may be usefully added to the batch. The batch should be sizegraded to provide typical brickmaking batches. Preferably the materialsare sized at least to pass 8 mesh. Materials which are sized somewhatfiner, say 14 mesh, provide good density and smoother surfaces.Continuous grind brickmaking batches and gap grind brickmaking batchesare both suitable according to this invention. To these batchingredients are added volatilized silica, for example, 2 to 10% byweight of the batch, a small quality of organic materials which providebinding and lubricating properties to the mix and a small quantity of aliquid vehicle such that the total of organic material and liquidvehicle comprise 13 to 18 volume percent of the batch. The batch is thenprocessed in a pug mill-extruder having a water-cooled barrel and die todeair and extrude a continuous column. The column is preferably in theform of a sheet. The sheet-like column is cut to form constant volumebillets having the desired volume of the reformed ceramic shapes. Thebillets are then pressed or reformed at low pressures into the desiredshape. After drying and firing the shapes are recovered as ceramicallybonded articles.

As already stated, this process is particularly applicable to themanufacture of slide gates. These gates are machined to ±0.001 inches offlatness and to provide a polished surface. Ceramic shapes prepared bythe above described process require much less machining to achieve theflatness specification.

Further features and other objects and advantages of this invention willbecome clear from the following detailed description.

High alumina refractory slide gates may be prepared according to thisinvention from a mixture of tabular and calcined alumina ground to pass14 mesh. To this is added small quantities of organic lubricants andbinders which include, for example, dextrine, CMC (carboxymethylcellulose), PVA (polyvinyl alcohol), polyethylene oxide (Polyox WSR 301)and a small quantity of water. Two percent by weight dextrine works verywell as the organic lubricant and binder. With proper selection, theorganic materials and vehicle may comprise between 13 and 18% by volumeof the batch materials. To this is added a small quantity, for example,approximately 5% by weight of the batch volatilized silica. Volatilizedsilica is a form of silica recognized in the art as having an extremelyfine particle size. It is generally obtained as a by-product of thereduction of silica to form silicon alloys. The batch is premixed, forexample, in a wet pan and fed at a controlled rate preferably from anair-tight storage container to the pug mill extruder.

Pug mill-extruder typically having a mixing section connected to one endof an extrusion barrel. An auger lays along the bottom of the mixingsection and passes into the extrusion barrel. Attached to the other endof the extrusion barrel is the die.

Applicants have found it preferably if the pug mill-extruder has anextrusion barrel length to auger diameter of at least 4:1. The extrusionbarrel which contains the auger must be water-cooled and preferably ismade with a replaceable liner since the auger and liner are susceptibleto wear in this process. If the pug mill-extruder is not maintained atabout room temperature, heat produced from friction between the batchand the machine becomes too great, and causes premature surface drying,with subsequent surface cracking of the preforms. Secured to theextrusion barrel is a transition die which is a smooth fishtailconfiguration having a water jacket for cooling. A straight finish diehaving a water jacket is secured to the transition die to providesmoother surface and to allow shimming for adjustment of the columnthickness.

An extruded column 101/2" by 11/2" is handleable without deforming.However, the column may be cut into preforms or billets for slide gatesusing a "cookie cutter" type apparatus for punching out the blanks orbillets.

For making a slide gate with a hole in the middle thereof, the flatrectangular blank is first cut into a preform having a volume exactlyequal to the desired volume or the reformed part. The overall size ofthe preform is slightly smaller than the die size and the hole in thepreform is slightly larger than the hole in the finished part so thatthe preform will fit easily into the die and over the mandrel. The holecutter is sharpened from the inside and the exterior wall of the cutteris sharpened from the outside. This way a precision billet can beobtained. An integral spring loaded ejection plate serves to eject thepreform after a force of only one ton cut the blank into the preform.

After the billet is formed by cutting from the column, it may benecessary to permit the billet to stand in the air thus permitting thesurface of the billet to loose some moisture. A slight drying of thesurface enable easier release of the reshaped billet from the press.However, too much drying of the surface will result in a pressed shapethat has an undesirably crazed surface. Indeed, this is the reason forkeeping the column at room temperature during extrusion. Otherwise, thesurface of the billet cannot be kept from loosing too much moisture.

The preforms or billets are then placed in a press and pressed to sizeat a pressure of about 500 to 1000 psi. This is considerably less thanthe approximately 4000 psi necessary for dry pressing. The deairedpreform is relatively incompressible. The pressure in the pressingprocess merely reforms the billet to the dimensions of the press box.

Because the preform is fully densified and uniform in size, the finalthickness of the part can be controlled accurately. Further, thethickness of the part can be adjusted by shimming the extrusion die toincrease the thickness of the column. Since the preforms are cut bycutters that have a fixed area, changing the thickness of the columnchanges the total volume of the preform. Thus, weighing, hand-fillingand hand-leveling are eliminated.

After pressing to reform the billet, it is ejected, dried and fired.

Slide gates were made according to this process from a batch comprising95%, by weight, tabular and calcined alumina ground to pass 14 mesh, 5%volatilized silica, and based on these inorganic ingredients, 2%dextrine and 3 to 4% water. The organic binder (dextrine) and watercomprise 18%, by volume, of the entire batch. This batch was processedinto slide gates as described above. After firing the slide gates had anapparent porosity of 15.4% and a modulus of rupture at 2500° F. of 1880psi.

The process can be made entirely automatic in the sense that alloperations can be done mechanically, thus eliminating manual steps.Making the process fully automatic involves the integration of the threemain forming steps, extruding, preforming and repressing so that theycycle in unison. However, the process is particularly adaptable tomechanization and automation because no step requires particularly closeattention by workmen. Dry pressing requires close attention because ofthe variability of the compaction ratio (batch volume to shape volume).Also the workpieces, according to this method, i.e., column, billet orreformed shape each have sufficient "wet" strength for mechanicalhandling.

Having thus described our invention with the detail and particularity asrequired by the patent laws, what is desired protected by Letters Patentis set forth in the following claims.

We claim:
 1. A method of making a ceramic shape from a batch comprisingnonplastic materials comprising raw clay wherein said raw clay ispresent in an amount of less than 5% comprising the steps for:A.preparing a batch comprising nonplastic size graded ceramic materialscomprising less than 5% raw clay, 2 to 10%, by weight, volatilizedsilica; and 13 to 18 volume percent based on the entire batch, oforganic binders, lubricants, and liquid vehicle; B. processing the batchin a pug mill-extruder to deair and extrude a continuous column, saidpug mill-extruder having means for maintaining the temperature of theextrusion barrel of said extruder near room temperature therebymaintaining the surface of the column near room temperature while it isbeing extruded; C. cutting the column to form a billet; D. pressing thebillet to form a shape; E. drying and firing to recover a ceramicallybonded shape.
 2. A method of making a ceramic shape from a batchcomprising nonplastic materials comprising raw clay wherein said rawclay is present in an amount of less than 5% comprising the steps for:A.preparing a brickmaking batch comprising nonplastic size graded ceramicmaterials comprising less than 5% raw clay passing 8 mesh; 2 to 10% byweight volatilized silica; and 13 to 18 volume percent, based on theentire batch of organic binders and lubricants selected from the groupconsisting of dextrine, carboxymethyl cellulose, polyvinylalcohol,polyethylene oxide, and mixtures thereof, and water; B. processing thebatch in a pug mill-extruder to deair and extrude a continuous column,said pug mill-extruder having means for maintaining the temperature ofthe extrusion barrel of said extruder near room temperature therebymaintaining the surface of the column near room temperature while it isbeing extruded; C. cutting the column to form a billet; D. pressing thebillet to form a shape; E. drying and firing to recover a ceramicallybonded shape.
 3. The method according to claim 2, in which the billet isallowed to stand prior to pressing just long enough to loose sufficientsurface moisture to enhance release of the shape after pressing but notlong enough to cause crazing of the surface upon pressing.
 4. The methodaccording to claim 1, wherein the batch comprises nonplastic ceramicmaterials comprising less than 5% raw clay at least all passing 8 mesh.